The present invention relates to an electric-control-type throttle valve apparatus for controlling the amount of intake air in an internal combustion engine.
Japanese Patent Publication 177534/1996 discloses a throttle valve control apparatus including a throttle valve body, a throttle valve installed in the air intake path of the throttle body via a rotatable shaft, an actuator for driving the throttle valve via a plurality of gears, and a detection means for detecting the rotation angle of the throttle valve.
The plurality of gears of this apparatus is composed of the a first gear fixed to the shaft of the throttle valve, the a third gear fixed to the rotation shaft of the motor used as the actuator, and a second gear engaged between the first and third gears. This gear arrangement can increase the speed reducing ratio, which can allow for fine control of the opening of the throttle valve.
Although the opening of the throttle valve can be finely controlled by obtaining a large gear ratio in the above-mentioned conventional throttle valve apparatus, it is not stated in the above-reference publication how to determine the characteristics of the motor, or the gear ratio with which the torque of the motor is transmitted to the throttle valve, in order to realize a desired operation speed of the throttle valve.
In the above-described conventional apparatus, the throttle valve is opened or closed by a motor. If the opening or closing action of the throttle valve does not respond quickly to the motion of the acceleration pedal operated by a driver, the driver will notice the lag between the operations performed by the driver itself and changes in the operational state of the internal combustion engine. However, in an electric-control-type throttle valve apparatus, it is necessary to provide a force applying means for quickly returning the position of the throttle valve to a predetermined opening which may affect event of a problem with the fail-safe operation. Therefore, with such force applying means, the operation speed of the throttle valve cannot easily be increased, and so it is important to adequately determine the force applied from the force applying means, the performance of the motor, and the speed reducing ratio of the rotation speed of the motor relative to that of the throttle valve.
If this conventional throttle valve apparatus is applied to a direct injection engine in which fuel is directly injected in each cylinder, the following problems will likely occur.
In a conventionally used port injection engine in which fuel is injected into an air intake pipe, the engine is operated near the theoretical air to fuel ratio of 14.7. On the other hand, a direct injection engine is operated in a wide range of valves of air to fuel ratio from 14.7 (theoretical ratio) to more than 40 (superlean ratio). Fuel burning near the theoretical air to fuel ratio is referred to as a uniform mixture charge burning state, and fuel burning at an air to fuel ratio higher than the theoretical ratio is referred to as a stratified charge burning state. It is easy to realize a stratified charge burning state in a direct injection engine because fuel is directly injected into a cylinder. FIG. 12 is a diagram showing the relationship between the fuel burning modes and operation states of an engine. The stratified charge burning mode is performed below an engine rotation speed of approximately 3000 rpm.
In implementing those burning modes, it is necessary to open the throttle valve wider in the stratified charge burning state than in the uniform mixture charge burning state. Therefore, when the operation of the engine is changed from the stratified charge burning state to the uniform mixture charge burning state, the throttle valve is driven in the valve closing direction. FIG. 4A and FIG. 4B show changes in time of the actuating amount of an acceleration pedal and changes in time of the opening of the throttle valve corresponding to the changes of the actuating amount of the acceleration pedal, respectively.
As shown in FIG. 4B, the throttle valve is widely opened in the stratified charge burning state, and it is driven initially in the valve closing direction when the operation of the engine is switched to the uniform mixture charge burning state. If the time necessary for the switching operation is long, the switching operation between the two burning states cannot be smoothly carried out, and the output power of the engine rapidly changes. Consequently, a shock caused by the switching operation is transmitted to the passengers and the driver of the vehicle, which degrades both the operationality of the vehicle and the comfort of riding in the vehicle.
On the other hand, if the throttle valve is driven at a high speed, it is also necessary to rotate the motor driving the throttle valve at a high speed. In such high speed operations, the higher the speed the motor is rotated at, the larger the counter-electromotive force for braking the rotation of the throttle valve becomes. Therefore, there may be a large current that is beyond the permitted value for the switching elements used in a drive circuit for driving the motor. It is then necessary to use switching elements with a higher permitted current value for the drive circuit of the motor. However, switching elements with the required larger permitted current cannot be always acquired. Even when switching elements with the required larger permitted current can be acquired, such elements are very expensive and are unsuitable for use in a vehicle. As another means for restricting the value of the current flowing in the drive circuit of the motor below the permitted current value, it is also possible to provide a current limiting circuit in the drive circuit. However, this tends to increase the production cost, and if the provided current limiting circuit breaks down, it is possible that the increased current cannot be kept below the permitted current value. Thus, such a solution does not offer a sufficient fail-safe function.